Game machines, such as a slot game machine and a pinball game machine, have been devised to perform presentation that arouses the player's visual sense, auditory sense, or feeling so as to enhance the player's interest. Especially for performing presentation that arouses the player's visual sense, a movable body, such as a movable accessory, may be provided in the game machine. A motor is used for driving such a movable body. For moving the movable body by a predetermined moving amount at a predetermined speed in accordance with the presentation, a motor control device is used, for example. The motor control device receives a control command including a target speed and a target rotation amount from an upper-level device, for example, and controls the motor in accordance with the command.
In order to enhance the player's interest, a large-sized movable accessary may be mounted in the game machine. Driving such a movable accessary requires a motor having high torque. There may thus be used a direct current (DC) motor that is cheaper than a stepping motor, and is smaller in size than the stepping motor to exert the same torque. In this case, along with the DC motor, a rotation angle sensor, such as a rotary encoder, is used which outputs a detection signal in every rotation of the DC motor at a predetermined rotation angle. The motor control device counts the number of times it received a detection signal from the rotation angle sensor, and can thereby determine whether or not the moving amount of the movable body has reached the predetermined moving amount. When the moving amount reaches the predetermined moving amount, the motor control device stops the rotation of the DC motor.
In the case of driving the movable body by using the motor such as the DC motor, a mechanical mechanism constituting the movable body may be locked to restrict the move of the movable body. In such a case, by the motor forcibly moving the movable body, an overload may be applied to the motor to generate heat, causing a breakdown or ignition. For detecting such an abnormality, there has been proposed a technique of detecting a locked state of the motor when a state continues for a certain period of time where a duty ratio of a pulse signal outputted from a pulse width modulation (PWM) circuit is the maximum value (e.g., see Japanese Patent Application No. 2002-347296).
However, the motor may rotate backward due to external application of unexpected force to the movable body. While the moving body is not completely locked, the movable body may become only able to move at a moving speed lower than an intended moving speed due to an increase in friction caused by some factor. As a result, the motor can only rotate at a lower rotation speed than the target speed. In such a case, with the motor rotating itself, the state where the duty ratio of the pulse signal is the maximum value does not continue, which may result in failure to detect such an abnormality in the above technique.